A 30 GHz laser frequency comb for high-precision radial velocity calibration and exoplanet searches*

Radial velocity precision at and below the 1 m/s level is necessary for the detection of earth-mass exoplanets within the habitable zones of M-dwarfs. Laser frequency combs provide a dense and absolute array of frequencies ideal for the in-situ calibration of high-resolution astronomical spectrographs. However, the challenges of generating large mode spacings, wide optical bandwidth, and the operational robustness suitable for remote facilities have prevented their widespread use. Overcoming these obstacles, we built a 30 GHz frequency comb spanning 800 nm to 1300 nm and achieved long term, on-sky, 1.5 m/s rms radial velocity precision with the near-infrared Habitable-Zone Planet Finder spectrograph at the 10 m Hobby-Eberly Telescope in Texas.

Our frequency comb is generated through robust, fiber integrated electro-optic modulation of a single frequency laser and subsequent supercontinuum generation in a highly nonlinear fiber and a nano-photonic device. All frequencies are traceable to a GPS disciplined atomic clock. The frequency comb has been running continuously since May 2018. We detail the architecture, performance, and long-term operation of the comb.